1. Field of the Invention
The invention relates generally to radar PPI displays and is particularly adaptable to marine collision avoidance radar systems.
2. Description of the Prior Art
Collision avoidance radar systems are known in the art that presents radar target information on a plan position indicator (PPI) display along with synthetically generated symbology to provide collision avoidance information to the ship officers with respect to threats of collision between own ship and the radar detected target ships. Such a system is disclosed in U.S. Pat. No. 3,725,918, issued Apr. 3, 1973 to Fleischer et al entitled "Collision Avoidance Display Apparatus For Maneuverable Craft" and assigned to the present assignee.
In systems exemplified by that disclosed in said U.S. Pat. No. 3,725,918, generally the radar target data is provided on a cathode ray tube PPI display with synthetically generated symbology written during the deadtime of the PPI azimuth sweeps. Such systems utilize the radar trigger to initiate the azimuth sweeps and the radar antenna position data to position the sweeps in azimuth in synchronism with the rotating antenna. In typical systems the radar antenna executes a complete rotation every one to three seconds. Therefore in such systems the display is updated at this slow rate, presenting viewing difficulties which are particularly aggravated in daylight. Such displays provide the conventional PPI appearance of a radially directed azimuth sweep emanating typically from the center of the screen and slowly rotating about the center thereof in synchronism with the slow rotation of the radar antenna "painting" the radar targets as the sweep slowly rotates across the azimuth positions thereof.
As discussed above, in such systems synthetic symbology is drawn during the flyback or deadtime of the azimuth sweeps. Such an arrangement generally requires separate electronic circuitry and display deflection means such as deflection coils or plates for the display of the synthetic symbology. Such separate display presentation provisions increases the cost and size of the equipment as well as decreases its reliability. Additionally, separate deflection means for the radar data and for the synthetic symbology introduces critical alignment and registration problems between the information provided by the separate deflection means.
Because of the above described arrangement for such displays, radar target as well as synthetic symbology brightness is limited resulting in viewing difficulties particularly in daylight. Since the azimuth sweep flyback time during which the synthetic symbology is written is of very short duration, high writing rates are required in order to draw the significant amount of symbology required in such displays. The high writing rate coupled with the exceedingly slow refresh rate of the PPI results in a display that is difficult to view. The high writing rate required necessitates wide bandwidth deflection amplifiers thereby increasing the power dissipation of the system.
Additionally, in these prior art systems, a brightness problem and a concomitant power dissipation disadvantage existed with respect to writing and radar data. When the range scale of such systems was decreased a faster azimuth sweep rate was required, causing a diminution in radar target brightness relative to that provided at the larger range scale. This not only resulted in a display that was difficult to view because of dimly drawn symbology but also resulted in non-uniformly bright radar displays on the various range scales of the system. This non-uniformity in brightness with respect to the radar data coupled with the brightness problems discussed above with respect to the synthetic symbology provided an overall display wherein the intermixed symbols were undesirably non-uniformly bright.
In the prior art systems the differing beam sweep rates required on different range scales of the system not only resulted in non-uniform brightness, but also in excessive power dissipation. In such systems it was necessary to provide deflection amplifier bandwidths in accordance with the fastest sweep rates required. The power supplies for the wide bandwidth amplifiers necessarily provided sufficient power corresponding to the bandwith. When slower sweep rates were utilized on the larger range scales of the system the resulting power dissipation was greater than required.